The present invention relates to a broad-band amplifier used for a broad-band input amplifier stage such as an oscilloscope, etc. and particularly to a broad-band amplifier which is capable of compensating a drift effect caused by scattering characteristics of elements, a temperature change, etc.
In a broad-band amplifier according to a prior art used for analog electronic measuring instruments, a broad-band frequency signal ranging from DC to high frequency needs to be amplified at a low drift. Especially, when such a conventional broad-band amplifier is employed for a vertical input circuit such as an oscilloscope, etc., it has to have both a high input impedance to alleviate a load effect on a signal source to be measured and a low output impedance to broaden a band of a post amplifier. Furthermore, there has been a demand for a stable broad-band amplifier with a small drift caused by scattering characteristics of elements or a temperature change.
As is disclosed in Japanese Patent Publication No. 61-1926 Official Gazette and Japanese Utility Model No. 61-15622 Official Gazette, it has been known that input signals are separated into high-frequency signal components and low-frequency signal components to be amplified and the thus amplified high-frequency and low-frequency signals are added. Thus, according to said system, signals are separated into high-frequency signals and low-frequency signals and the thus separated signals are respectively amplified by two different amplifiers to accomplish a stable broad-band amplification. However, a field-effect transistor employed in this system requires the application of a self-bias. Since a self-bias requires a high resistance value, thermal noise tends to be generated in the resistance, which causes a distorted output waveshape.
In order to solve the aforementioned problem, a broad-band amplifier which utilizes a source-follower field effect transistor and which is not greatly affected by a drift caused by scattering characteristics of elements and an ambient temperature change has been developed as is disclosed in Japanese Patent Publication No.63-15764 Official Gazette. Such a broad-band amplifier will briefly be explained by way of FIG. 5. In the figure, a gate and a drain of a field-effect transistor 31 are respectively connected to an input terminal 32 and a positive voltage source, the source being connected to a negative voltage source via a constant current source consisting of a bipolar transistor 33 and a resistor :34. A source of the field-effect transistor 31 is further connected to a base of a transistor 35. A potential divider 36 disposed on the input terminal 32 and another potential divider 37 disposed on the output side have almost the same dividing ratio, divided input signals and output signals being provided to an operational amplifier 38. Error signals of the operational amplifier are sent to a base of the transistor 33 via a low-pass filter 39 consisting of a resistor and a condenser.
In FIG. 5, an input signal to be applied on the input terminal 32 is amplified by a broad-band amplifier having the field-effect transistor 31 and a transistor 35 (amplification factor: about 1). A signal passage including the aforementioned input potential divider 36, output potential divider 37 and operational amplifier 38 stabilizes a circuit of a broad-band amplifier at low frequency and also improves accuracy thereof. The operational amplifier 38 compares a part of a signal from the input potential divider 36 with a part of an output signal from the output potential divider 37 and applies an error signal between them on the base of the transistor 33. When a gate source voltage Vgs of the field-effect transistor 31 or a base emitter voltage V.sub.BE changes in accordance with difference in operating condition and as a result, an output voltage and an input voltage become different from each other, a part of an output signal of the operational amplifier 38 being fed back to the operational amplifier 38, the difference between an output voltage and an input voltage being automatically corrected by the high-gain operational amplifier 38 and the current source transistor 33. More specifically, the output voltage is corrected in such a manner that it automatically becomes equal to the input voltage regardless of the change in Vgs and V.sub.BE.
The broad-band amplifier described in FIG. 5 stabilizes a circuit within a low frequency band by changing a drain current Id to change the gate source voltage Vgs of the source-follower field-effect transistor 31. Therefore, it does not require an auto-bias resistor and thus, thermal noise is not generated. However, when an emitter current flowing through the transistor 33, that is, a drain current Id of the field-effect transistor 31 changes, a mutual conductance g.sub.m which constitutes transfer characteristics of the field-effect transistor 31 also changes. Thus, the change in the mutual conductance g.sub.m affects amplification characteristics of the source-follower field-effect transistor 31 within a high-frequency band.